Computer viruses initiated by electronic vandals are now responsible for the loss of untold hours of labor. In many cases, these viruses are spread as attachments to electronic mail (e-mail). When a recipient opens e-mail initiated by a vandal, and then opens the attachment, the virus is unleashed to do its damage. For this reason, experienced computer users are sometimes reluctant to open e-mail attachments.
Consequently, electronic vandals often go to great lengths to give their e-mail a veneer of legitimacy, often by fraudulently stating the identity of its originator. For example, a virus may gain access to the address book of a first computer user, and propagate itself, under the assumed identity of the first computer user, to a number of recipients who have come to trust the integrity of e-mail received from the first computer user. Still, when the virus is carried by an attachment to e-mail, the recipient may open the e-mail and inspect the attachment outwardly without taking undue risk. Upon inspection, the odd nature of malevolent attachments, for example their use of odd file types, may alert the recipient to the true purpose of the e-mail, and the recipient may discard the e-mail without opening the attachment and therefore without sustaining any damage.
Unfortunately, vandals have now developed viruses that may be carried directly by e-mail, and do not require transport in an attachment. When a victim receives such an e-mail that fraudulently purports to originate from a trusted source, the victimized recipient may instinctively open the e-mail and thereby contract the virus. Thus, there is a need for a method of authenticating e-mail—verifying that the e-mail indeed originates from the source whose identity it bears, and may therefore be presumed virus-free—which does not require that the recipient open the e-mail.